Stent grafts are used to bridge a defect in the vasculature of a patient and can be deployed into the vasculature by endovascular techniques. This requires that the device can be constrained into a small delivery device and be able to expand or be expanded when release within the vasculature.
Where there is a side branch artery to the vasculature it may be necessary to provide an aperture in the stent graft, known as a fenestration, to enable access from a deployed stent graft to that side branch artery. Such a fenestration may be reinforced with a peripheral circular ring stitched to the graft material around the fenestration. It is also desirable in some situations to provide a side branch stent graft extending through the fenestration and into the side branch artery.
To obtain a good seal of the branch stent graft within the fenestration an inflatable balloon can be used to expand the branch stent graft into the fenestration and for this purpose the reinforcing ring must be able to resist expansion of its diameter. At the same time the ring must be resilient so that it can be distorted into its deployment configuration but when released expand back to its circular configuration. In this specification the term resilient when used in relation to a wire used to manufacture a reinforcing ring refers to a wire which is substantially inextensible but which has a spring function so that when distorted and released returns to its original configuration.
Generally such reinforcing rings are manufactured from a metal known as a shape memory metal such as, but not restricted to, a nickel titanium alloy known as Nitinol. To form a ring of a shape memory metal the desired final shape is formed from a wire on a former and then the wire on the former is heated above a temperature which sets the wire in the new shape. Upon cooling the ring holds its formed shape and can be distorted and resiliently returns to the formed shape.
It is also desirable that the position of reinforcing ring on a stent graft when the stent graft is deployed within the vasculature be able to be visualised. Nitinol, from which reinforcing rings have been constructed is not radiopaque and hence it has been necessary to use radiopaque markers adjacent the reinforcing ring to denote its position.